Breads, cookies, vegetables, fermented foods such as fermented soybeans and kimchi, etc. have been conventionally sold in a state wrapped in paper or plastic bags. Though paper bags have high air permeability and moisture permeability, they are disadvantageous in not permitting their contents to be seen. On the other hand, plastic bags permit contents to be seen, but they are disadvantageous in not having sufficient air permeability and moisture permeability, and extremely deteriorating the flavor and texture of foods.
To obtain a plastic film permitting contents to be well seen, while having high air permeability and moisture permeability, apparatuses for forming large numbers of fine pores in a plastic film are known. For example, JP 6-71598 A discloses an apparatus for producing a microporous film comprising a means for supplying a long plastic film, a first roll (pattern roll) having large numbers of high-hardness, fine particles having sharp edges and Mohs hardness of 5 or more fixed to its rolling surface, a second roll (metal roll) having a flat rolling surface and rotatable in an opposite direction to that of the first roll, a pressure-adjusting means disposed near both ends of either one roll for adjusting a pushing force to the long plastic film, and a means for applying high voltage to the first roll, either one or both of the first and second rolls being movable in their arrangement direction. The first and second rolls are arranged in parallel, and the long plastic film passing through a gap therebetween is provided with large numbers of fine pores by large numbers of high-hardness, fine particles on the first roll.
However, when large numbers of fine pores are formed in a plastic film (not shown) as thick as about 8-100 μm passing through a gap between a pattern roll 10 and an anvil roll (metal roll) 20, a large load is applied to the pattern roll 10 and the anvil roll 20, so that both rolls 10, 20 tend to be bent, resulting in a gap G, which is wider in a transverse center portion than in both side edge portions, as shown in FIG. 16. Fine pores formed by an uneven gap G have different opening diameters and depths between a transverse center portion and both side edge portions of the film, failing to obtain a microporous plastic film Fa having transversely uniform air permeability.
To prevent the bending of a pattern roll 10 and a anvil roll 20, backup rolls were arranged above the pattern roll 10 and/or under the anvil roll 20. However, because of large numbers of high-hardness, fine particles attached to a rolling surface of the pattern roll 10, soft-surface rubber rolls, etc. should be used as backup rolls, failing to sufficiently prevent the bending of the pattern roll 10 and the anvil roll 20.
JP 6-328483 A discloses an apparatus for calendaring a film of a thermoplastic polymer or rubber by six rolls arranged in substantially parallel, a sixth roll being arranged just under first to fifth rolls; and comprising a crossing means for inclining the fifth roll by a small angle. However, the angle of the fifth roll inclined by the crossing means is constant, and the calendaring apparatus does not have a driving means for obtaining a desired inclination angle. Accordingly, even if this calendaring apparatus were used to form fine pores in a plastic film, fine pores having various sizes and distributions cannot be formed uniformly in a width direction.